The flying into power lines and other types of wire has proved to be a major hazard not only for military aircraft but also for commercial and general aviation. In addition to wire strike accidents, the stress of low-level flying is magnified by the possibility of flying into an unseen and often uncharted wire. Thus not only is the accident itself traumatic, the flying of low level missions either for the military or for surveillance, inspection or rescue missions causes great anxiety among pilots.
By way of background, data shows that many helicopters involved in accidents have been classified as wire-strike accidents.
Wires present particular risks to helicopters because helicopters often are flown at low altitudes and at off-airport sites for takeoff, landing, and other purposes. Some of the types of wires that pilots may encounter are power transmission lines, guy wires used to support other objects, such as towers and communication cables.
When crossing rivers and valleys, power transmission lines can be as high as several hundred feet above ground level (AGL). Guy wires that support towers may be almost invisible to pilots, even if the general location of the wires is known. Pilots' ability to see wires is affected by dirty windscreens, light conditions, the obscuring effects of terrain and changes in visual perspective that occur during climb and descent. In addition, accurately judging distance from unmarked wires is nearly impossible.
Data also shows that 30% of wire strike accidents resulted in at least one fatality, and 18% resulted in serious injuries. Many wire strike accidents occur at night, more often causing fatalities and destroying the helicopter.
By way of example, one fatal accident occurred in dark-night conditions near Littleton, Colo. A Bell 407, being operated as an emergency medical services (EMS) flight, struck wires after takeoff and fell inverted to the ground. All four people on board were killed.
The U.S. National Transportation Safety Board (NTSB) said, in the final report on the accident, that the helicopter had been flown to the site of the fatal automobile accident from the northeast and that the pilot had circled the area before conducting a north-to-south approach to the landing. Lights from emergency response vehicles on the ground illuminated the landing area.
After the patient was on board, the pilot, a former U.S. Army helicopter pilot with more than 4,000 hours of helicopter flight time, conducted a departure to the south and circled to the right, remaining at a low altitude. The NTSB report said that the pilot's company policy, which was promulgated through documents and training, included landing-zone departure procedures, which instructed the pilot to climb straight ahead in a near-vertical climb to a minimum of 300 feet AGL before turning.
About 630 feet west of the takeoff point, the helicopter struck unmarked power lines. Existence of the power lines was unknown to the fire-rescue on-scene commander, and the light conditions prevented the pilot from seeing anything outside the lighted area. The power lines were supported by two towers 622 feet apart and located on a riverside golf course. The unmarked power lines did not meet obstruction-lighting criteria and were not marked. In addition, they were not depicted on sectional or topographic maps.
Although the helicopter was equipped with a wire-strike protection system designed to help protect the helicopter in the event of inadvertent flight into horizontally strung wires or cables consisting of a windshield deflector, an upper cutter/deflector and a lower cutter/deflector, these measures proved insufficient to prevent the crash.
Note that in addition to reviewing aeronautical charts and talking with pilots who are familiar with the area, a pilot who is about to begin low-altitude operations first should conduct a reconnaissance flight at a higher altitude.
Nevertheless, those precautions may not be adequate for detecting all wires.
Wires are difficult to see, partly because of the way the human eye functions and partly because of the effects of some backgrounds and light angles in camouflaging wires. The eye starts to lose its visual acuity at three degrees off-center. Unless one is looking straight at a wire, one is unlikely to see it. Obviously, night operations are severely limited.
The movement of wires in the sunlight and changing sunlight patterns can obscure wires. Wires also may be difficult to see because as they age, their color often changes. For example, copper wires oxidize with age, acquiring a greenish color that makes them difficult to distinguish from grass and trees in the background. The exact location of specific wires may change throughout the day because of fluctuating ambient temperatures, which may cause wires to sag or to tighten within several hours. Also, sagging wires may be blown by wind. In addition, optical illusions involving wires are common.
The following are examples of some of the systems on the market to help identify power lines and prevent aircraft from colliding with them.
Spherical markers sometimes are used to mark power lines, communications lines, and guy wires at airports or helicopter approach areas and at locations where wires cross rivers and canyons. These markers often are orange, but in some instances, others colors are used because they may be more visible, depending on the surrounding terrain. Some spherical markers used on electrical power lines are designed to glow as a result of the power line's electrical field. Other spherical markers are patterned for improved visibility or equipped with flashing lights.
Several wire-detection systems developed in recent years have been installed in aircraft to warn pilots when they are near wires. For example, Safe Flight Instrument Corp.'s Powerline Detection System sense the electromagnetic field generated by live electrical power lines and emits an audible alert through the aircraft's audio system, a clicking sound that increases in frequency as the aircraft is flown nearer to a live electric power line, i.e., a line carrying electric current. The system then illuminates a red warning light in the cockpit. The warnings are provided regardless of whether the helicopter is approaching the power line from above, below, or at an oblique angle. The system does not alert pilots to where the wires are and does not work with wires that are not live. Moreover, the distance at which the electromagnetic field can be sensed is sometimes only 30 feet.
The Hellas helicopter laser radar system developed by Dornier, a subsidiary of the European Aeronautic Defence and Space Co., uses eye-safe laser radar to scan the environment for wires and other flight obstacles and provides optical signals and acoustic signals to warn pilots about their presence. However, laser systems often miss the presence of wires.
During flight tests in which the locations of power lines were included in a computer database, Honeywell's Enhanced Ground-proximity Warning System (EGPWS), which warns pilots of rising terrain and obstacles that are 100 feet or more above ground level (AGL), delivers warnings of approximately 30-seconds to pilots that their helicopters were approaching the power lines. However, this system does not work for uncharted or unmapped wires.
Thus, although the system is capable of delivering the warnings, its database lacks the required information, which generally has not been available from utilities and other organizations that control wires.
In summary, because most rotary wing and fixed wing aircraft are not equipped with wire-detection systems and because not all wires are marked, wires continue to present risks.
What has become apparent is that cutters and deflectors are insufficient to prevent wire strike accidents. Moreover, although in the past large amounts have been invested in radar and LIDAR systems to illuminate and detect wires, be they power lines or trip wires, there is an unacceptably high failure to detect wires. In addition, because these systems are active systems from which the position of the aircraft can be ascertained, military use is ill advised. Thus for military applications there is a requirement for a passive wire strike avoidance system that is effective while maintaining a degree of stealth.
From the point of view of commercial and general aviation, there is likewise a requirement for a robust wire strike avoidance system that is inexpensive and effective.